Re: [pbs-devel] [PATCH v2 proxmox-backup 4/4] fix #5331: garbage collection: avoid multiple chunk atime updates

["Fabian Grünbichler" <f.gruenbichler@proxmox.com>]

On March 10, 2025 12:16 pm, Christian Ebner wrote:
> Reduce the number of atime updates on the same chunk by logically
> iterating over image index files, following the incremental backup
> logic. By inserting paths for encountered images during
> `list_images` using the GroupedImageList structure, the iteration
> happens now for the same image filenames in the same image namespace
> and group in a order based on the snapshot timestamp. For each image,
> keep track of the encountered chunk digests, and remember these as
> seen for the next snapshot. Chunks which have been encountered in the
> previous image index, but are not present anymore are removed from
> the list after each image, in order to reduce memory footprint.
> 
> Fixes: https://bugzilla.proxmox.com/show_bug.cgi?id=5331
> Signed-off-by: Christian Ebner <c.ebner@proxmox.com>

technically this could be split into two parts - one for the iterating
change, one for the performance optimization? with the latter part maybe
being combined with patch#3 and the order switched around? ;)

> ---
> changes since version 1:
> - Use pre-existing datastore iterator helpers, following the logic other
>   datastore operations take.
> - Chunks are now remembered for all archives per snapshot, not just a
>   single archive per snapshot as previously, this mimics more closely
>   the backup behaviour.
> 
>  pbs-datastore/src/datastore.rs | 117 +++++++++++++++++++++++----------
>  1 file changed, 84 insertions(+), 33 deletions(-)
> 
> diff --git a/pbs-datastore/src/datastore.rs b/pbs-datastore/src/datastore.rs
> index fdbb33a98..a80343d9b 100644
> --- a/pbs-datastore/src/datastore.rs
> +++ b/pbs-datastore/src/datastore.rs
> @@ -25,7 +25,7 @@ use pbs_api_types::{
>      MaintenanceMode, MaintenanceType, Operation, UPID,
>  };
>  
> -use crate::backup_info::{BackupDir, BackupGroup};
> +use crate::backup_info::{BackupDir, BackupGroup, BackupInfo};
>  use crate::chunk_store::ChunkStore;
>  use crate::dynamic_index::{DynamicIndexReader, DynamicIndexWriter};
>  use crate::fixed_index::{FixedIndexReader, FixedIndexWriter};
> @@ -970,10 +970,10 @@ impl DataStore {
>          ListGroups::new(Arc::clone(self), ns)?.collect()
>      }
>  
> -    fn list_images(&self) -> Result<Vec<PathBuf>, Error> {
> +    fn list_images(&self) -> Result<HashSet<PathBuf>, Error> {
>          let base = self.base_path();
>  
> -        let mut list = vec![];
> +        let mut list = HashSet::new();
>  
>          use walkdir::WalkDir;
>  
> @@ -1021,7 +1021,7 @@ impl DataStore {
>                  if archive_type == ArchiveType::FixedIndex
>                      || archive_type == ArchiveType::DynamicIndex
>                  {
> -                    list.push(path);
> +                    list.insert(path);
>                  }
>              }
>          }
> @@ -1071,6 +1071,7 @@ impl DataStore {
>          &self,
>          index: Box<dyn IndexFile>,
>          file_name: &Path, // only used for error reporting
> +        touched_chunks: &mut TouchedChunks,
>          status: &mut GarbageCollectionStatus,
>          worker: &dyn WorkerTaskContext,
>      ) -> Result<(), Error> {
> @@ -1081,6 +1082,12 @@ impl DataStore {
>              worker.check_abort()?;
>              worker.fail_on_shutdown()?;
>              let digest = index.index_digest(pos).unwrap();
> +
> +            // Avoid multiple expensive atime updates by utimensat
> +            if touched_chunks.insert(*digest) {
> +                continue;
> +            }
> +
>              if !self.inner.chunk_store.cond_touch_chunk(digest, false)? {
>                  let hex = hex::encode(digest);
>                  warn!(
> @@ -1107,43 +1114,87 @@ impl DataStore {
>          status: &mut GarbageCollectionStatus,
>          worker: &dyn WorkerTaskContext,
>      ) -> Result<(), Error> {
> -        let image_list = self.list_images()?;
> -        let image_count = image_list.len();
> -
> +        // Iterate twice over the datastore to fetch images, even if this comes with an additional
> +        // runtime cost:
> +        // - First iteration to find all index files, no matter if they are in a location expected
> +        //   by the datastore's hierarchy
> +        // - Iterate using the datastore's helpers, so the namespaces, groups and snapshots are
> +        //   looked up given the expected hierarchy and iterator logic
> +        //
> +        // By this it is assured that all index files are used, even if they would not have been
> +        // seen by the regular logic and the user is informed by the garbage collection run about
> +        // the detected index files not following the iterators logic.
> +
> +        let mut unprocessed_image_list = self.list_images()?;
> +        let image_count = unprocessed_image_list.len();
> +
> +        // Optimize for avoiding updates of chunks atime in same group for consecutive
> +        // snapshots multiple times.
> +        let mut touched_chunks = TouchedChunks::new();
> +        let mut processed_images = 0;
>          let mut last_percentage: usize = 0;
>  
> -        let mut strange_paths_count: u64 = 0;
> -
> -        for (i, img) in image_list.into_iter().enumerate() {
> -            worker.check_abort()?;
> -            worker.fail_on_shutdown()?;
> -
> -            if let Some(backup_dir_path) = img.parent() {
> -                let backup_dir_path = backup_dir_path.strip_prefix(self.base_path())?;
> -                if let Some(backup_dir_str) = backup_dir_path.to_str() {
> -                    if pbs_api_types::parse_ns_and_snapshot(backup_dir_str).is_err() {
> -                        strange_paths_count += 1;
> +        let arc_self = Arc::new(self.clone());
> +        for namespace in arc_self
> +            .recursive_iter_backup_ns(BackupNamespace::root())
> +            .context("creating namespace iterator failed")?
> +        {
> +            let namespace = namespace.context("iterating namespaces failed")?;
> +            for group in arc_self.iter_backup_groups(namespace)? {
> +                let group = group.context("iterating backup groups failed")?;
> +                let mut snapshots = group.list_backups().context("listing snapshots failed")?;
> +                // Sort by snapshot timestamp to iterate over consecutive snapshots for each image.
> +                BackupInfo::sort_list(&mut snapshots, true);
> +                for snapshot in snapshots {
> +                    for file in snapshot.files {
> +                        worker.check_abort()?;
> +                        worker.fail_on_shutdown()?;
> +
> +                        let mut path = snapshot.backup_dir.full_path();
> +                        path.push(file);
> +
> +                        let index = match self.open_index_reader(&path)? {
> +                            Some(index) => index,
> +                            None => continue,
> +                        };
> +                        self.index_mark_used_chunks(
> +                            index,
> +                            &path,
> +                            &mut touched_chunks,
> +                            status,
> +                            worker,
> +                        )?;
> +
> +                        unprocessed_image_list.remove(&path);
> +
> +                        let percentage = (processed_images + 1) * 100 / image_count;
> +                        if percentage > last_percentage {
> +                            info!(
> +                                "marked {percentage}% ({} of {image_count} index files)",
> +                                processed_images + 1,
> +                            );
> +                            last_percentage = percentage;
> +                        }
> +                        processed_images += 1;
>                      }
> +                    touched_chunks.reset();
>                  }
>              }
> -
> -            if let Some(index) = self.open_index_reader(&img)? {
> -                self.index_mark_used_chunks(index, &img, status, worker)?;
> -            }
> -
> -            let percentage = (i + 1) * 100 / image_count;
> -            if percentage > last_percentage {
> -                info!(
> -                    "marked {percentage}% ({} of {image_count} index files)",
> -                    i + 1,
> -                );
> -                last_percentage = percentage;
> -            }
>          }
>  
> +        let strange_paths_count = unprocessed_image_list.len();
>          if strange_paths_count > 0 {
> -            info!(
> -                "found (and marked) {strange_paths_count} index files outside of expected directory scheme"
> +            warn!("found {strange_paths_count} index files outside of expected directory scheme");
> +        }
> +        for path in unprocessed_image_list {
> +            let index = match self.open_index_reader(&path)? {
> +                Some(index) => index,
> +                None => continue,
> +            };
> +            self.index_mark_used_chunks(index, &path, &mut touched_chunks, status, worker)?;
> +            warn!(
> +                "Marked chunks for unexpected index file at '{}'",
> +                path.to_string_lossy()
>              );
>          }
>  
> -- 
> 2.39.5
> 
> 
> 
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> 


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